Eliza-
I would also suggest that as you practice, continuously look at the orientation of your left hand with respect to the angle of the controller itself. This is to assure that your QCF and HCB are true to angle, and not offset +/- 45 degrees.
What also helps for me, is how I hold the old-style stick. I don't know how tall you are, but i'm just shy under 6', and what I do (no innuendo intended) is simply put the controller between my legs so that it is held by my lower, inner thighs and the upper corners of the controller stop at the knee. Then I cross my caves, basically bracing the controller in one position. This assures that it stays in one place, but I maintain my level of comfort while sitting up straight. This maximizes my dexterity and agility for any fighter, Schmup, what have you.
Spending time in practice mode really is the best solution overall though. Really, practice all the quarter circle and half circle motions... ALOT. Practice your QCF x 2 AB, BC, CD, AC, BD moves as well as your QCF, HCB, motions too. These motions are very common for SNK DM's and SDM's.
Timing... as fast as you can accurately pull off each move, that's all I can say. The CPU thinks fast, it'll catch every button press you do.
Hope this helps.